1. Field of the Invention
This invention relates to a semiconductor memory device and a method of writing data into the semi-conductor memory device. More particularly, this invention relates to a write operation in a magnetic random-access memory (MRAM).
2. Description of the Related Art
MRAM is the generic name for solid-state memories which use the magnetization direction of a ferro-magnetic material as an information recording carrier and can rewrite, hold, and read recorded information at any time.
MRAM memory cells generally have a structure where a plurality of ferromagnetic materials are stacked one on top of another. Information is recorded by causing whether the relative arrangement of magnetizations of a plurality of ferromagnetic materials forming a memory cell is parallel or anti-parallel to correspond to binary information “1” or “0.” Recorded information is written by reversing the magnetization direction of the ferromagnetic material in each memory cell by a current magnetic field.
MRAM, which is completely nonvolatile, can be rewritten more than 1015 times. In addition, it enables nondestructive reading and therefore requires no refresh operation. Accordingly, the read cycle can be shortened. Moreover, it is more resistant to radiation than charge accumulation memory cells. As described above, MRAM has more advantages in function than conventional semiconductor memories using dielectrics. It is expected that the integration per unit area of MRAM and the write and read times are roughly the same as those of DRAM (Dynamic Random Access Memory). Thus, it is expected that MRAM will be applied to an external recording unit for portable devices, LSI embedded packages, or the main memory of a personal computer, taking full advantage of nonvolatility.
In the MRAM now on the way to practical use, a magnetic tunnel junction (hereinafter, abbreviated as MTJ) is used for memory cells. MTJ has been disclosed in, for example, “IEEE International Solid-State Circuits Conference 2000 Digest Paper,” TA7.2. MTJ is made up of a three-layer film, composed mainly of a ferromagnetic layer/an insulating layer/a ferromagnetic layer. Current tunnels through the insulating films. The resistance value of the junction varies in proportion to the cosine of a relative angle of the magnetization of both ferromagnetic metal layers. Then, the resistance value of the junction takes the maximal value when the directions of magnetizations of both ferromagnetic layers are anti-parallel. This is a tunnel magneto-resistive effect. One type of MTJ structure is to hold data, making use of the difference in retentivity between the two ferromagnetic materials. Another known type of MTJ is a spin valve structure type which is such that antiferromagnetic material is provided next to one ferromagnetic material to fix the magnetization direction to improve the magnetic field sensitivity or reduce the write current. The spin valve structure has been disclosed in, for example, “Japanese Journal of Applied Physics,” Vol. 36, 1997, p. 200.
In the conventional MRAM, an MTJ element is provided at the intersection of two wires crossing at right angles. Then, a current of about 1 mA is caused to flow in the two wires. With the resultant one of the magnetic fields generated by the wires, the data is written into the MTJ element.
Another known method is to write data into a GMR element by causing current to flow in two wires crossing at right angles, while raising the temperature of the free layer in the GMR element using FeMn as a material for a free layer. This method has been disclosed in, for example, “Journal of Applied Physics,” Vol. 87, 2000, p. 6403. In this method, current of about 5 mA is caused to flow in a sense line, which raises the temperature of the free layer to the Neel temperature or higher. Then, FeMn, an antiferromagnetic material, is transformed so as to have ferromagnetic-state. In this state, current of about 200 mA is caused to flow in a word line. As a result, the magnetic fields produced by the sense line and word line reverse the direction of spin of the free layer, thereby writing the data into the GMR element.
As described above, tremendous research effort has been directed toward using MRAM as one of the new generation of memory devices. The conventional MRAM, however, has the problem of getting larger in chip size.